Method and apparatus for detecting dust particles by partial embedment in a layer whose surface is deformed by the particles and measuring radiation scattered by the particles with a schieren optical system

ABSTRACT

A method and apparatus for detecting and measuring the number and/or size of dust particles on a support plate having a planar surface. The method comprising flowing a liquid layer over the support to wet the particles and controlling the layer thickness so that particles form deformations in the layer surface due to surface tension of the liquid and then measuring,by means of a Schlieren or phase contrast optical system, the amount of light scattered by those particles causing the deformations. The measured intensity is then compared with calibration curves derived empirically from measurements of dust particles of known size embedded in liquid layers of different known thicknesses to determine the number and/or size of the particles under measurement. The apparatus comprises a dust proof chamber into which the support plate with the particles embedded in a liquid layer is introduced, the chamber including a light source for illuminating the liquid layer and a Schlieren optical system for measuring the light scattered from the particles. The optical system includes a photoelectric transducer which is connected to an evaluation system not forming part of the housing to provide an indication of the number and/or size of the particles under measurement.

United States Patent 1 Mast et al. v

[ METHOD AND APPARATUS FOR DETECTING DUST PARTICLES BY PARTIAL EMBEDMENTIN A LAYER WHOSE SURFACE IS DEFORMED BY THE PARTICLES AND MEASURINGRADIATION SCATTERED BY THE PARTICLES WITH A SCHIEREN OPTICAL SYSTEM [75]Inventors: Fred Mast, Zuzwil/Sg; Renato Rossi,

Zurich; Ulrich La Roche, Zurich;

Jean A. Knus, Zurich, all of Switzerland [73] Assignee: Gretag Aktiengesellschaft,

Regensdorf, Switzerland 221 Filed: Feb. 9, 1972 21 App]. No.: 224,721

[30] I Foreign Application Priority Data Feb. 17, 1971 Switzerland2314/71 [52] U.S. Cl 356/102, 250/222 PC, 235/92 PC, 356/38, 356/129,356/208 [51] Int. Cl. G01n 15/02, G0ln 21/46 [58] Field of Search356/36, 38, 102, 356ll03,l04, 120,129, 207, 210, 212;

250/222 PC, 218; 235/92 PC; 350/13 [56] References Cited UNITED STATESPATENTS 2,918,216 12/1959 Shapiro 235/92 PC 2,950,648 8/1960 Rhodes, Jr.350/13 2,977,847 4/1961 Meyer-Arendt 350/13 3,335,413 8/1967 Glenn, Jr.350/161 2,850,239 9/1958 Polanyt et al 235/92 PC 3,526,461 9/1970Lindahl et al 356/38 2,604,809 7/1952 1 Mitchell 356/120 [451 Oct. 23,1973 OTHER PUBLICATIONS The Measurement of Unresolved Single ParticlesPhase Microscope; Osterberg et al.; JOSA; Vol. 40 No. 2, Feb. 50; pg.64-72 Primary ExaminerRonald L. Wibert Assistant Examiner-4V. P. McGrawAttorney-Ralph E. Parker et al.

[57] ABSTRACT A method and apparatus for detecting and measuring thenumber and/or size of dust particles on a support plate having a planarsurface, The method comprising flowing a liquid layer over the supportto wet the particles and controlling the layer thickness so thatparticles form deformations in the layer surface due to surface tensionof the liquid and then measuring,by means of a Schlieren or phasecontrast optical system, the amount of light scattered by thoseparticles causing the deformations. The measured intensity is thencompared with calibration curves derived empirically from measurementsof dust particles of known size embedded in liquid layers of differentknown thicknesses to determine the number and/or size of the par ticlesunder measurement. The apparatus comprises a dust proof chamber intowhich the support plate with the particles embedded in a liquid layer isintroduced, the chamber including a light source for illuminating theliquid layer and a Schlieren optical system for measuring the lightscattered from the particles. The optical system includes aphotoelectric transducer which is connected to an evaluation system notforming part of the housing to provide an indication of the numberand/or size of the particles under measurement.

16 Claims, 3 Drawing Figures 1 METHOD AND APPARATUS FOR DETECTING DUSTPARTICLES BY PARTIAL EMBEDMENT IN A LAYER WHOSE SURFACE IS DEFORMED BYTHE PARTICLES AND MEASURING RADIATION SCATTERED BY THE PARTICLES WITH ASCHIEREN OPTICAL SYSTEM FIELD OF INVENTION This invention relates to amethod of detecting particles, for example dust particles, in which theparticles are deposited on a support, the support together with thedeposited particles being brought into the path of a beam of radiationand the light scattered by the particles being detected by an opticalsystem.

PRIOR ART Known methods of this kind in which the optical systemincludes a viewing system and a light source for illuminating thesupport for the dust particles, can give information solely as to thequantity of scattered light, and not the number and size of the dustparticles. It is also known to scan the support with a microscope todetermine the number and size of the dust particles but this method isextremely time-consuming and complicated.

BRIEF SUMMARY OF THE INVENTION The invention seeks to overcome thesedisadvantages by embedding the particles in a layer of liquid whosethickness is less than the maximum height of the deposited particles,and interposing the layer containing the embedded particles as a lightcontrol layer in a Schlieren or phase contrast optical system.

The layer or film used for embedding the particles need be liquid onlyduring the embedding operation and should have maximum wetting power inthe liquid phase. The embedding substance may be in a solid phase or bebrought into such a phase before and/or after the embedding operation.The particles affect the measurement made by the optical system only ifthey deform or break through the surface of the layer of liquid, i.e.,if their height is greater than the thickness of the layer of liquid.The size of the particles can thus be determined by variation of thethickness of the layer of liquid.

The invention further relates to apparatus for performing the abovemethod, and comprises a plateshaped support for the particles, a sourceof radiation, means for spreading a layer of liquid on the support, andmeans for introducing the support together with the layer of liquid andparticles embedded therein as a light control layer into the path of theradiation produced by said source so that the radiation reflectedfromsaidparticles is measured by a phase contrast optical system.

BRIEF DESCRIPTION OF THE DRAWINGS A preferred embodiment of theinvention will now be explained in detail with reference to theaccompanying drawings wherein:

FIG. 1 illustrates a method of detecting particles in accordance withthe invention;

FIG. 2 is a graph of intensity of light received by the optical systemshown in FIG. 1 versus depth of liquid on the support; and

FIG. 3 is a sectional view of the apparatus according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, afilm 1 of a wetting agent is spread over a support 3. The film of liquid1 is illuminated by a light source 9. The beam of light L bounded by therays 6 and 8 is reflected by the surface of the film of liquid. Thereflected beam L is directed towards the pupil 7 of an optical viewingsystem 10 by means of a Schlieren optical system S including a lens 4and a bar and slot system 5. This arrangement is very sensitive to dustparticles having a height H greater than the film thickness d. FIG. 1shows a dust particle 2 embedded in the film of liquid 1. After beingwetted by the liquid, the dust particle 2 produces a surface deformationin the deposited film as a result of surface tension, the deformationaround the particle 2 having a diameter D which is considerably greaterthan the height H of the dust particle.

The position of the support 3 relative to the optical system S and thelight source 9 is arranged so that those rays, for example the ray 6,which are incident to undeformed parts of the film of liquid 1 arereflected on to the bar 5 of the Schlieren optical system and are maskedby that bar. These rays 6 do not therefore reach the viewing system 10.In contradistinction thereto, those rays, for example the ray 8, whichare incident=to the zone of the film of liquid 1 deformed by the dustparticle 2 and are thus reflected past the bar 5. In the viewing system10, these rays 8 appear as diffraction images 8 and 8 in the form ofbright strips outside the bar 5 and symmetrical thereto.

A thin mirror coating applied to the support 3 may alternatively be usedto reflect the rays emitted by the light source 9. The Schlieren opticalsystem may also be so arranged that it is not the rays reflected by thelayer of liquid that are viewed, but the rays which pass through thelayer of liquid. The dust particle light scatter effect, which is knownper se, is considerably a'mplified by the interaction between dustparticles and the wetting layer of liquid and by the resultingdeformations occurring at the surface of the liquid. The amplificationfactor is about 104 to 10 The diffraction spectrum of the surfacedeformations of the layer of liquid 1 produced by the dust particlesappears in the plane of the pupil 7 of the viewing system. These surfacedeformations would appear as spots of light to the eye direct, i.e. byviewing the layer of liquid 1 through the viewing system 10 without theinterposition of the Schlieren optical system. S. The larger the surfacedeformations, the brighter these spots of plane of the pupil 7 is thus adirect indication of the number and/or size of the embedded dustparticles. The intensity of the scattered light produced by each surfacedeformation of diameter D as a result of an embedded dust particle ofheightI-I is approximately proportional to the square of half thediameter of the deformation. Practical experiments have shownthat theratio of the square of half the diameter of the deformation to thesquare of the height of the dust particle is approximately 10 to 10: 1,i.e., there is an effective amplification of the scattered light of 10to 10 asa result of the deformation of the layer of liquid. Thisincreased amount of scattered light is measured by the Schlieren opticalsystem.

With a dust particle of given height H and a liquid thickness d of zero,the diameter D of the deformation of the liquid surface and hence thescattered light intensity I is also zero, increases with increasinglayer thickness, reaches a maximum value, and returns to zero for d H.This relationship is shown by curve B in FIG. 2. When dust particles ofdifferent sizes are deposited on the support 3, the scattered lightintensity I increases with increasing liquid layer thickness, alsoreaches a maximum value, and then drops again. This relationship isshown by the curve A in FIG. 2. If the thickness of the layer of liquidis known, the size of the dust particles can be deducted from the totalscattered light intensity. Since the scattered light intensity isdirectly proportional to the number and/or brightness of the spots oflight appearing to the eye viewing the layer of liquid 1 directly i.e.,without the bar the size and number of dust particles can readily beobtained by counting the number of spots of light for different layerthicknesses d. Each count gives the number of those particles whoseheight H is greater than the layer thickness d. This somewhat tediousmethod can be automated by using electronic scanners. Since scanners ofthis kind are very expensive, the following discloses a preferred methodby means of which the size and number of the dust particles i.e., theirdistribution curve can be determined more easily and cheaply. Thismethod makes use of the relationship shown in FIG. 2 between thescattered light intensity I, the layer thickness d and the number andthe size of the dust particles.

The equation applies to the measured total scattered light of intensityI; H denoting the maximum particle height, z(H) the particle density inthe height interval dh and L(H,d) denotes the intensity of the scatteredlight from a particle of height H against the layer thickness d. If thefunctions L(H,d) are known, the above equation can be solved withrespect to z(H) by measuring the total scattered light intensity I fordifferent layer thicknesses d, i.e., by determining the function I(d).The functions L(H,d) are determined by measuring the scattered lightintensity from particles of known height for different layerthicknesses.

Thus by measuring the scattered light as a function of the thickness ofthe layer of liquid it is possible to obtain the number of dustparticles as a function of their size provided that the scattered lightintensity is known per particle of a given size as a function of thelayer thickness. The scattered light intensity per particle of a givensize for different layer thicknesses is determined by coating acalibration plate having elevations of a specific height at specificplaces with layers of liquid each of a different thickness and measuringthe total scattered light intensity for each layer thickness. A group ofcalibration curves can be obtained in this way, and will have a similarappearance to the curve B in FIG. 2. These calibration curves may be fedas functions to a computer which solves the above equation by referenceto these calibration curves, and to the layer thicknesses and theintensities of the particles being examined. The layer thicknesses canbe determined by an interferometer or they can alternatively be readilydetermined when the layer of liquid is applied to the support. Thislatter method is possible, for example, if an applicator bar mounted forparallel displacement at a predetermined distance from the support 3 isused for applying the layer. In these conditions, the liquid is movedacross the support into the space between the support and the applicatorbar. It is known that the resulting layer thickness is dependent uponthe different material constants of the liquid, the speed of movement ofthe applicator bar, and the geometry of the liquid meniscus formingbetween the support and the applicator bar. Thus, for example, to applylayers of liquid of different thicknesses either the distance betweenthe support and the applicator bar can be kept constant and the speed ofthe applicator bar varied, or else the speed can be kept constant andthe distance varied. The layer thickness is in every case calculable orempirically determinable.

The apparatus shown in FIG. 3 comprises a housing 20 containing thelight source 9, the Schlieren optical system S (FIG. 1) and aphotoelectric transducer behind the pupil 7 and the bar 5 of theSchlieren optical system, an electronic evaluating system 11 beingconnected to the transducer 100. The housing 20 has a projection 21 intowhich the plate-shaped support 3 together with the dust particlesdeposited thereon can be slid in the direction of arrow E. The supportplate 3 is connected to a cover 22 so as to be dust-tight, for exampleis glued thereto. After the support 3 has been inserted into theprojection 21, the cover 22 is withdrawn from the support 3, for exampleby means of a plunger 23 slidably mounted in the housing 20. In theexemplified embodiment illustrated, the space 24 between the innersurface of the plunger and the cover 22' is sealed by means of an O-ring25, so that the cover can be removed from the support plate 3 byevacuation of the space 24. The plunger 23 together with the coveradhering thereto is then brought into the position shown in brokenlines. Removal of the cover has the advantage that its outer surface,which is usually not dustfree, adheres to the plunger inner surface sothat the dust particles on the outer surface of the cover are notliberated in the housing 20 where they might impair the measurement.After removal of the cover, the support plate 3 is coated with a liquidhaving a viscosity between 2 and 6 centipoises by an applicator bar 12,different layer thicknesses being applied step-by-step. Alternativelythe support plate 3 can be coated with liquid just once during ameasuring operation, namely with a layer of liquid of the maximumrequired thickness, and then the thickness of the layer can be reducedeither by mounting the support vertically and allowing the layer ofliquid to flow away, or by mounting the support horizontally andallowing the layer of liquid to evaporate. In either case, however, thethickness of the layer of liquid must be determined, this being possiblein known manner, for example by an interferometer or by the supportplate 3 containing calibration particles of known size at predeterminedpoints. Particularly suitable liquids are oils, more particularlysilicone oils. The support plate 3 is any substrate having an opticallyplane surface.

A variant of the above-described embedding of the dust particles in theliquid has proved particularly advantageous for practical performance ofthe method described. In this variant, the support 3 for the dustparticles is constructed in the form of a cassette which can he slidinto the apparaus shown in FIG. 3 after the dust particles have beenapplied. A layer of a thermo plastic material of predetermined thicknessis disposed on that surface of the support on which the dust particlesare to be collected. After the dust particles have been deposited onthis layer, the support together with the thermoplastic layer is heated,the latter becoming liquid and wetting the dust particles. After coolingof the support, the layer of liquid solidifies so that any particles ofdust applied subsequently for example unintentionally can no longersubject the surface to deformation and the closed cassette can be storedfor any length of time.

The advantage of the method described over known methods comprises theamplification of to 10 as compared with direct scattered lightmeasurements, this effect being due to the surface deformation of thelayer of liquid, and the possibility of exact measurement of the size ofthe dust particles, since the only particles that give rise to surfacedeformations are those whose diameter is larger than the thickness ofthe layer of liquid.

The method according to the invention and the systems operating inaccordance therewith may be used preferably wherever information isrequired concerning the presence of solid particles in a gaseous medium.The dust suspended in space is not measured directly; instead, it is thedust deposited on a support in a given interval of time. Measurements ofthis kind are used more particularly for monitoring areas which are tobe kept as dust-free as possible.

What is claimed is:

1. A method of detecting particles comprising the steps of depositingthe particles on the surface of a support member, wetting the particlesby flowing a liquid layer over said surface, the depth of said layerbeing less than the maximum height of said particles, projecting a beamof radiation onto said layer and measuring the radiation scattered bysaid particles by means of a Schlieren optical system.

2. A method according to claim 1, including empirically determining therelationship between different quantities of scattered radiationmeasured by said optical system for different predetermined number ofparticles of a given size embedded in layers of different thicknessesand relating the measurements to the scattered radiation received bysaid optical system from a layer having particles embedded therein todetermine the number of particles and their size.

3. A method according to claim 2, including mounting said support memberhorizontally, allowing the liquid to evaporate and measuring theradiation scattered by said particles at different predeterminedthicknesses of said layer as said liquid evaporates.

4. A method according to claim 1 wherein the step of wetting theparticles on the surface of said support member is performed by flowingthe liquid layer over said surface after the particles have beendeposited on said surface.

5. A method according to claim 1 wherein the step of wetting theparticles on the surface of said support member is performed by flowinga layer of liquid over the surface and which has a viscosity within therange of from 2 to 6 centipoises.

6. A method according to claim 1 wherein the step of wetting theparticles on the surface of said support member is performed by flowinga layer of silicone oil over the surface.

7. A method according to claim 1 wherein the step of wetting theparticles on the surface of said support member is performed by coatingsaid surface with a layer of thermoplastic material, heating saidthermoplastic layer subsequent to deposition thereon of the particles tobe detected to a temperature sufficient to liquify the layer and wet theparticles, and then permitting the layer to cool and re-solidify.

8. Apparatus for detecting particles comprising a source of radiation, aSchlieren optical system for receiving radiation, a support member, alayer formed on a surface of said member to receive and wet dust partivcles, and means for positioning said support member relative to saidsource and said optical system so that said layer acts as a radiationcontrolling layer for said optical system.

9. Apparatus according to claim 8, including means for spreading aliquid layer over the surface of said support member to wet dustparticles deposited thereon.

10. Apparatus according to claim 8, wherein said layer is ofthermoplastic material.

11. Apparatus for detecting particles comprising a housing, a lightsource and a Schlieren optical system within said housing, a supportmember having a planar surface, means for introducing said surface intosaid housing and means for applying a liquid layer to said surface towet particles thereon, said optical system being positioned relative tosaid layer and said source so that light scattered by the deformationscaused by the particles in said layer is measured by said opticalsystem.

12. Apparatus according to claim 11, wherein said optical systemincludes a photoelectric device for receiving light scattered by saidparticles and said apparatus further includes an evaluating systemcoupled to said photoelectric device for determining the number ofparticles of a predetermined size.

13. A method of detecting particles comprising the steps of embeddingsaid particles in a substance lying on the surface of a support memberwhilst said substance is in a liquid phase, the depth of said liquidsubstance being less than the maximum height of said particles,projecting light into the surface of said substance and measuring bymeans of a Schlieren optical system the variation in the intensity oflight resulting from deformations in the surface of said substancecaused by said particles.

14. A-method according to claim 13 wherein the step of embedding saidparticles in the substance is performed by use of a substance which isnormally in a liquid phase.

15. A method according to claim 13 wherein the step of embedding saidparticles in the substance is performed by coating the surface of saidsupport member with a layer of thermoplastic material and heating saidthermoplastic layer subsequent to deposition thereon of the particles tobe detected to a temperature sufficient to liquify the layer.

16. A method according to claim 13 wherein the step of measuring thevariation in intensity of light resulting from deformation in thesurface of said substance caused by said particles is performed byprojecting the light through the substance and the support member tosaid Schlieren optical system.

1. A method of detecting particles comprising the steps of depositingthe particles on the surface of a support member, wetting the particlesby flowing a liquid layer over said surface, the depth of said layerbeing less than the maximum height of said particles, projecting a beamof radiation onto said layer and measuring the radiation scattered bysaid particles by means of a Schlieren optical system.
 2. A methodaccording to claim 1, including empirically determining the relationshipbetween different quantities of scattered radiation measured by saidoptical system for different predetermined number of particles of agiven size embedded in layers of different thicknesses and relating themeasurements to the scattered radiation received by said optical systemfrom a layer having particles embedded therein to determine the numberof particles and their size.
 3. A method according to claim 2, includingmounting said support member horizontally, allowing the liquid toevaporate and measuring the radiation scattered by said particles atdifferent predetermined thicknesses of said layer as said liquidevaporates.
 4. A method according to claim 1 wherein the step of wettingthe particles on the surface of said support member is performed byflowing the liquid layer over said surface after the particles have beendeposited on said surface.
 5. A method according to claim 1 wherein thestep of wetting the particles on the surface of said support member isperformed by flowing a layer of liquid over the surface and which has aviscosity within the range of from 2 to 6 centipoises.
 6. A methodaccording to claim 1 wherein the step of wetting the particles on thesurface of said support member is performed by flowing a layer ofsilicone oil over the surface.
 7. A method according to claim 1 whereinthe step of wetting the particles on the surface of said support memberis performed by coating said surface with a layer of thermoplasticmaterial, heating said thermoplastic layer subsequent to depositionthereon of the particles to be detected to a temperature sufficient toliquify the layer and wet the particles, and then permitting the layerto cool and re-solidify.
 8. Apparatus for detecting particles comprisinga source of radiation, a Schlieren optical system for receivingradiation, a support member, a layer formed on a surface of said memberto receive and wet dust particles, and means for positioning saidsupport member relative to said source and said optical system so thatsaid layer acts as a radiation controlling layer for said opticalsystem.
 9. Apparatus according to claim 8, including means for spreadinga liquid layer over the surface of said support member to wet dustparticles deposited thereon.
 10. Apparatus according to claim 8, whereinsaid layer is of thermoplastic material.
 11. Apparatus for detectingparticles comprising a housing, a light source and a Schlieren opticalsystem within said housing, a support member having a planar surface,means for introducing said surface into said housing and means forapplying a liquid layer to said surface to wet particles thereon, saidoptical system being positioned relative to said layer and said sourceso that light scattered by the deformations caused by the particles insaid layer is measured by said optical system.
 12. Apparatus accordingto claim 11, wherein said optical system includes a photoelectric devicefor receiving light scattered by said particles and said apparatusfurther includes an evaluating system coupled to said photoelectricdevice for determining the number of particles of a predetermined size.13. A method of detecting particles comprising the steps of embeddingsaid particles in a substance lying on the surface of a support memberwhilst said substance is in a liquid phase, the depth of said liquidsubstance being less than the maximum height of said particles,projecting light into the surface of said substance and measuring bymeans of a Schlieren optical system the variation in the intensity oflight resulting from deformations in the surface of said substancecaused by said particles.
 14. A method according to claim 13 wherein thestep of embedding said particles in the substance is performed by use ofa substance which is normally in a liquid phase.
 15. A method accordingto claim 13 wherein the step of embedding said particles in thesubstance is performed by coating the surface of said support memberwith a layer of thermoplastic material and heating said thermoplasticlayer subsequent to deposition thereon of the particles to be detectedto a temperature sufficient to liquify the layer.
 16. A method accordingto claim 13 wherein the step of measuring the variation in intensity oflight resulting from deformation in the surface of said substance causedby said particles is performed by projecting the light through thesubstance and the support member to said Schlieren optical system.